Question

why is calcium carbide used in holme's signal

Answer 1

Actually, packets containing calcium carbide and calcium phosphide are pierced and thrown into water which then react with water to form C2H2 and PH3.

CaC2 + H2O → Ca(OH)2 + C2H2 (1)

Ca3P2 + H2O → Ca(OH)2 + PH3 (2)

P2H4, by product of the reaction of calcium phosphide with water, catches fire immediately and ultimately burns PH3 and C2H2 also.

This acts as a signal for other ships/ aeroplanes that they need some sort of help.

Answer 2

Containers containing calcium carbide andcalcium phosphide are pierced and thrown in the sea when the gases evolved burn and serve as a signal. It is also used in smoke screens.